1. Field of the Invention
This invention relates to a system for aerially transmitting an ultrasonic pulse, and more particularly to a system of aerially transmitting an ultrasonic wave having a thickness ultrasonic mode, narrow in angle of direction, in a clear state (noiseless state) and to a transmitter and receiver used in the system.
2. Prior Art
Conventionally, a system of transmitting an ultrasonic pulse by a transmitter and receiving the ultrasonic pulse propagated through the air by a receiver, detecting the pulse and obtaining an electrical signal corresponding to the envelope curve of the ultrasonic pulse has been employed as a system for aerially transmitting an ultrasonic pulse. In order to hold the ultrasonic pulse to be transmitted accurate with respect to time in industrial application of the system, it is necessary to obtain an optimum value of mechanical Q inherent in the transmitter and receiver; but it was difficult to manufacture the transmitter and the receiver by the prior art without reducing the mechanical Q. As a result, the ultrasonic pulse generated by the conventional transmitter produces complicated vibrations with accompanying long after-vibrations as shown in FIG. 2A and consequently, it is very difficult for the receiver to receive the pulse and to cause it to be driven in terms of an electrical means from the receiver while holding the pulse accurate with respect to time.
Namely, FIG. 2A is an explanitory view schematically showing the ultrasonic pulse which the piezoelectric element of the conventional ultrasonic transmitter generates by one driving pulse. In the figure when a driving pulse is applied to the conventional transmitter through a pulse transformer, the element is heavily excited by a piezoelectric effect, and emits an ultrasonic wave of a specified frequency (fo) in the air because of resonance characteristic of the piezoelectric element. The ultrasonic wave emitted in this manner produces heavy vibrations when a driving pulse is applied to the piezoelectric element, and even after the driving pulse disappears, not only after-vibrations continue for a long time when the mechanical Q is unnecessarily high, but also vibrations at the end face of the thickness piezoelectric element spread through the casing slightly later than the vibrations at the front end face and come over to the front end face, creating reverberation and echo thereon and interfering in the spread of the pulse. Accordingly, it is considered difficult in practical use to send out an ultrasonic pulse corresponding to the driving pulse with excellent damping characteristics. But when consideration is given to advantages obtainable both from the characteristics of the ultrasonic pulse far lower in propagation rate than light, electric wave, laser, etc. and from superiority of the pulse in directivity to an ordinary sound wave, the use of the ultrasonic pulse as a short distance signal transmission means in the air not only provides highly accurate measurement and control, but also finds very useful application to all other fields of industry. Also, although it is known that the characteristics of an ultrasonic wave makes it possible for the ultrasonic wave to be reduced in angle of direction by a small ultrasonic piezoelectric element; if it is possible to derive an ultrasonic pulse narrow in angle of direction from a thickness piezoelectric element without sacrificing such characteristics of the ultrasonic wave, it is not only possible to make useful application of the characteristics of the ultrasonic pulse, but also possible to make such aerial transmission of an ultrasonic pulse between movable bodies lying adjacent to obstacles, as has heretofore been considered impossible. It is needless to explain that the usefulness and increased application of such aerial transmission will be limitless.
This invention has been worked out after careful study of the above-described disadvantages inherent in the prior art of aerial ultrasonic transmission systems. The invention has made it possible to provide industrial applications of an ultrasonic pulse which corresponds to a signal pulse for driving a piezoelectric element which is excellent in damping characteristic, and has further enabled aerial transmission of the ultrasonic pulse. Namely, the invention has made it possible to aerially transmit an ultrasonic pulse by disposing the ultrasonic transmitter and receiver, matched and the same in structure, which are provided by the invention and which will become apparent from a description that follows, in an opposed relation with each other on a straight line as shown in FIG. 1, applying a driving pulse to a piezoelectric element in a transmitter having a suitable Q, vibrating the piezoelectric element in one cycle with increased amplitude as shown in FIG. 2B. As a result the driving pulse applied to the piezoelectric element is brought into corresponding relation with the vibration of the ultrasonic pulse generated by the driving pulse at a ratio of substantially 1:1 by reducing the time constant of an envelope curve of the subsequent damped oscillation of the piezoelectric element 1 and emitting the ultrasonic pulse by the transmitter S and receiving the pulse by the receiver R, thereafter converting the pulse into an electric signal. The area of transmission in which the ultrasonic pulse is transmitted on the aerial transmission system of an ultrasonic pulse, according to the invention, can be made very small like the area of space shown by X in FIG. 1, which fills two areas of space, one being the area of space in which the transmitter S emits an ultrasonic pulse small in angle of direction and the other being the area of space in which the receiver R having the same structure as the transmitter S, disposed in an opposed relation with the transmitter S, can sense the ultrasonic pulses emitted.